Crash in arenas_cleanup on linux x86-64

[Mike Hommey]

Hi,

I'm getting crashes in Firefox in some cases (only one test suite,
actually), and on Linux x86-64 only (not Linux x86, not Android ARM, and
not OSX x86 or x86-64).
They are a NULL deref in arenas_cleanup, in which the arena variable
seems to be NULL.
This happens with current dev branch. I had a hunch that I tested, and
it turns out commit cd9a134 is broken too and 154829d is not, which
makes cd9a134 the culprit.
I haven't looked why, though.

Cheers,

Mike